Conventionally, an ink-jet printer is known which includes an ink-jet head having a plurality of channels that discharge ink. By moving relatively the ink-jet head with respect to recording mediums such as a paper sheet, a cloth and the like and controlling an ink discharge, it is possible to output a two-dimensional image onto the recording mediums. It is possible to perform the ink discharge by using an actuator (piezoelectric type, electrostatic type, thermal deformation type and the like), or generating a bubble in ink in a tube by heating. Among others, an actuator of the piezoelectric type has advantages of a large output, possible modulation, high response, accepting any type of ink and the like, and is widespread in recent years.
As the actuators of the piezoelectric type, there are actuators that use a bulk piezoelectric material and actuators that use a thin-film piezoelectric material. The former has a large output, accordingly, can discharge a large liquid drop, but is large in size and costly. In contrast, the latter has a small output, accordingly, cannot output a large liquid drop, but is small in size and inexpensive. To realize a printer that has a high resolution (small liquid drop is enough), small size, and low cost, it can be said that it is appropriate to compose an actuator by using a piezoelectric thin film.
A piezoelectric thin film is sandwiched between a pair of electrodes (upper electrode, lower electrode) and located on a driven film (diaphragm) that composes an upper wall of a pressure chamber. With ink stored in the pressure chamber, by applying a voltage (drive signal) to the pair of electrodes to extend and shrink the piezoelectric thin film and vibrating the diaphragm, a pressure is given to the ink in the pressure chamber. In this way, it is possible to discharge the ink in the pressure chamber to outside. By arranging such actuators of the piezoelectric type in a lateral direction, an ink-jet head is composed.
As a method for discharging the ink from the pressure chamber, because of being effective in a stable ink discharge, a drawing-hitting method is widespread, in which a volume of the pressure chamber is temporarily expanded, thereafter, shrunk to discharge the ink. In the drawing-hitting method, a constant voltage (a standby potential at this time is V1) is applied to the actuator during a standby time to deform the diaphragm by a predetermined amount, the potential is dropped to V0 (<V1) at an ink discharge time, thereafter, returned to the standby potential V1, whereby the expansion and shrinkage of the volume of the pressure chamber are performed.
As a piezoelectric material used in the above actuator of the dielectric type, metallic oxides of perovskite type such as BaTiO3, Pb (Ti/Zr)O3 called PZT and the like are widespread. The actuator using a piezoelectric thin film is produced by depositing, for example, PZT on a substrate. It is possible to perform the deposition of PZT by using various methods such as a sputtering method, a CVD (Chemical Vapor Deposition) method, a sol-gel method and the like. In the meantime, crystallization of a piezoelectric material needs a high temperature. Accordingly, Si is often used for the substrate.
In the meantime, in recent years, the ink-jet printer is required to form a high-definition image at a high speed. Following this, an ink discharge waveform (drive waveform) of the ink-jet head is required to shorten a drive period per one pixel and perform multi-gradation.
However, if an interval between the ink discharges becomes short because of the high-speed drive, a reverberation of a pressure wave, which is generated in the pressure chamber by a discharge pulse applied immediately before, occurs and changes an ink discharge speed of the ink discharged next, so that it is impossible to stably discharge the ink. Because of this, in the high-speed drive, after the application of the discharge pulse, it becomes necessary to apply a cancel pulse for curbing the reverberation of the pressure wave to the actuator.
On the other hand, as to the multi-gradation, there is a method, in which a drive waveform is output by using an analog circuit, a shape of the drive waveform is changed to change a size of a discharged ink drop, so that the multi-gradation is achieve. But, in this case, a complicated and costly drive circuit becomes necessary.
Accordingly, in a patent literature 1, by applying a discharge pulse a plurality of times continuously in accordance with a natural vibration period of a pressure chamber, ink drops discharged per one pixel are increased and the multi-gradation drawing is achieved. In this method, because the discharge pulse is applied in accordance with the natural vibration period, influence of the reverberation becomes large, and it is necessary to apply the above cancel pulse for the high-speed stable drive.
Here, as waveforms of the cancel pulse, there is a pulse having a polarity opposite to the discharge pulse and a pulse having the same polarity as the discharge pulse. To quickly curb the reverberation, as described in a patent literature 2, it is effective to use the pulse as the cancel pulse having the polarity opposite to the discharge pulse. But, in a head using a thin-film piezoelectric element, a film thickness of the piezoelectric element is thin and an electric field (voltage per unit thickness) acting on the element is large. Because of this, in the drawing-hitting method, if the pulse having the polarity opposite to the discharge pulse is applied, there are concerns that the applied voltage exceeds a withstand voltage of the element, insulation breakdown of the element occurs, and reliability cannot be kept. Accordingly, in the head using the thin-film piezoelectric, as described in a patent literature 3, it is effective to use the pulse as the cancel pulse having the same polarity as the discharge pulse.